FIG 9.

Hypothetical transmission mode of Pneumocystis in animals and humans. In this mode, cysts (indicated by green, as seen with immunofluorescence staining) serve as the infectious form and mammalian hosts (e.g., rodents on the left and humans on the right) as the reservoir for infection; transmission occurs via the airborne route between immunocompetent hosts, between immunodeficient hosts, or between immunocompetent and immunodeficient hosts. The healthy population (indicated by three individuals) is substantially larger than the immunodeficient population (indicated by one individual).